Lamprey? Worm? Leech?

We found this strange creature in a brook up at Shedden Clough on May 7th at around midday. It has the appearance of an earthworm, but it is very pale, almost white. What really stumped us was the lamprey-like mouth which you can see in the first picture and which in the second picture seems to be attaching itself to the rocks under the water. Does anyone have any ideas as to what this could be?